fix: calculate role correctly (backport 2.9/26.2)

fix: calculate role correctly

We assume Role.User and ParticipationKind.Registered by default. The participation kind is not set in join success. We also calculate the guest state wrong.

The roomserver actually has two different Role types which are both exposed. The frontend uses the same type in all cases, but did not handle the case Role.Guest correctly. Since one of the role types lacks the Guest variant, it's not possible to handle it correctly with only one Role type in the frontend.

closes #3570 (closed)